This invention relates to combustion control in an internal combustion engine, and particularly to the control of dilution gas flow in response to the ratio of combustion chamber pressure to motored pressure as a function of crankshaft rotational angle.
Currently, control of internal combustion engines is based on sensing a set of variables such as coolant temperature, manifold pressure, engine speed and exhaust oxygen concentration and adjusting ignition timing, EGR rate and fuel flow to a prescribed calibration for a baseline engine. The problem with this approach is that, because of manufacturing differences and aging, the engine being controlled is not necessarily the same as the baseline engine used for reference. One approach to the solution of this problem requires extensive effort in the design of the engine to ensure both that all combustion chambers behave the same and that components will not significantly alter characteristics with aging. But even with such design and tight quality control in the manufacturing process, the required uniformity and stability over time of such engines cannot always be achieved.
A second approach to the solution of the problem is to implement a control system with the capability to adjust for these differences and changes. Such a control system is possible using combustion chamber pressure sensors and applying feedback control to ignition timing, dilution gas rate and fuel rate.
In a typical current engine control, the three controlled combustion parameters are spark timing (or fuel injection timing in a diesel engine), EGR rate and air/fuel ratio. The first affects the timing of the initiation of the combustion process while the latter two affect the speed and duration of the combustion process. In current practice, air/fuel ratio is generally controlled in closed loop by an exhaust oxygen sensor to produce a constant stoichiometric ratio for emission control by oxidizing and reducing catalysts in the exhaust system. Since the efficiency of one or the other catalyst falls rapidly as the air/fuel ratio strays even slightly from stoichiometric in either direction, this parameter must be strictly controlled and is not available for maximizing power or fuel efficiency. EGR is generally controlled by a combination of different parameters such as exhaust backpressure, engine coolant temperature, engine speed, throttle position or manifold pressure and has proven difficult to control accurately. Spark or injection timing is generally determined from a stored table addressed with engine speed and load parameters with additional retard, in some cases, in response to a knock, MAP or throttle movement detector.
Closed loop combustion control has been suggested in the prior art in various forms. LPP spark timing control systems have been proposed, in which spark timing is controlled to maintain a predetermined location of peak combustion pressure. This stability of the timing of peak combustion pressure has been found to produce MBT operation for many engine operating conditions. The location of peak pressure may be sensed using headbolts with an embedded piezoelectric material which responds to the stresses created in reaction to the pressure on the engine cylinder head. However, this approach has difficulties when the combustion charge is highly dilute or the engine is under light load. Some systems have been suggested which adjust ignition timing to control maximum absolute combustion pressure with respect to some predetermined reference level. There is, in addition, a suggestion, in the Greeves U.S. Pat. No. 4,449,501, issued May 22, 1984, that ignition timing be controlled to maintain the ratio of maximum combustion chamber pressure to maximum motored pressure in accordance with a stored table addressed by engine speed and load factors. These approaches, however, involve stored references determined for a baseline engine which may not be the desired reference for any particular engine at any given time. Other approaches have involved sensing flame position using ion gaps or engine speed variations. These approaches have drawbacks related, respectively, to cyclic variability in flame position and to difficulties with vehicle transients. If a measure of combustion pressure is available, several other approaches have been suggested. With unlimited computing capability, one might be able to perform a detailed heat release analysis or to obtain combustion duration to estimate air/fuel ratio. However, the computational requirements of these schemes prohibits their application for the foreseeable future.